Angles

An angle measures the amount of turn

Names of Angles

As the Angle Increases, the Name Changes:

Try It Yourself:

In One Diagram This diagram might make it easier to remember: Also: Acute, Obtuse and Reflex are in alphabetical order.

Also: the letter "A" has an acute angle.

Be Careful What You Measure

The smaller angle is an Obtuse Angle, but the larger angle is a Reflex Angle

So when naming the angles make sure that you know which angle is being asked for!

Positive and Negative Angles

When measuring from a line:

• a positive angle goes counterclockwise (opposite direction that clocks go)
• a negative angle goes clockwise

Parts of an Angle

The corner point of an angle is called the vertex

And the two straight sides are called arms

The angle is the amount of turn between each arm.

How to Label Angles

There are two main ways to label angles:

1. give the angle a name, usually a lower-case letter like a or b, or sometimes a Greek letter like α (alpha) or θ (theta)

2. or by the three letters on the shape that define the angle, with the middle letter being where the angle actually is (its vertex).

Example angle "a" is "BAC", and angle "θ" is "BCD"